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Polls have gotten closer in recent weeks, with Hillary Clinton and Donald Trump leading the final Des Moines Register poll, and Trump and Bernie Sanders leading a poll released Monday. Ted Cruz is a close second to Trump on the Republican side, and Marco Rubio has been gaining in recent weeks.

Mike Huckabee and Rick Santorum, winners of the 2008 and 2012 Iowa GOP caucuses, will try to pull an upset despite low poll numbers. Rand Paul, Ben Carson, Jeb Bush, Carly Fiorina, John Kasich, and Jim Gilmore are also hoping for a surprise, as is Martin O'Malley on the Democratic side.

Although the caucuses are really just a straw poll (Iowa won't officially assign its delegates until later in the process), it's commonly seen as an important first step to gauge the state of the race.